Friction control at sliding contact interface has been, and is still, a perpetual pursuit in the field of machine components and mechanical system technology. In some systems, relatively high friction is desirable, e.g., in brake system; however, in the vast majority of mechanical systems, friction reduction is the main goal. Lower friction usually translates to higher efficiency, better reliability and durability, all of which are desirable in machine components and mechanical systems, such as internal combustion engines (“ICE”), gearboxes and transmission systems in transportation vehicles for instance.
Currently, there are numerous approaches and strategies used for friction control, (mainly friction reduction). These include surface modification in terms of coatings or texturing and bulk material development and treatment. However, the most commonly used approach is by lubrication, either with grease or fluid lubricant. Lubricants are complex fluids consisting of basestock fluid and material specific functional additives, such as anti-wear (“AW”) and extreme pressure (“EP”) additives. These functional additives are designed to react with the surface materials under various contact conditions to form a thin surface layer, commonly referred to as tribochemical or boundary films. Effective lubrication of sliding interface is accomplished preferably through three structural components, namely lubricant fluid film, the tribochemical surface film and the near surface material. The overall friction, wear and other surface damage mechanisms occurring at lubricated sliding interface are all determined by the action of these three structural components.
More specifically, friction at a lubricated sliding interface is determined by the simultaneous shearing of one or more of the three structural elements depending on the operating lubrication regime. In the hydrodynamic and elastohydrodynamic regimes, the lubricant fluid film thickness is large enough to completely separate the two surfaces in sliding contact. Hence, under these regimes, the overall friction at the sliding interface is determined primarily by the shearing of the lubricant fluid film. Consequently, lubricant viscosity and other rheological properties govern the friction behavior. In the mixed regime, there is limited direct contact between the asperities on the contacting surfaces. In this regime, the overall friction is then determined by the shearing of the fluid film as well as the shearing of the few asperities in contact and the tribochemical surface film that may form on the asperities. For the case of the boundary lubrication regime, more interactions occur between the sliding surfaces. The near-surface materials often carry a substantial fraction of the contact load and more surface tribochemical films are formed in response to the severe contact conditions of this regime. Hence, in the boundary lubrication regime, the friction is determined by the shearing of all the three structural components, namely the fluid film, the tribochemical surface film and the near-surface material.
Of the three structural components of lubrication, the tribochemical surface film is the least well understood. The films are formed as a result of reaction between the material of surfaces in contact, additives in the oil, the base oil constituents and chemical species from the operating environments. Indeed, the films are best described as moieties of chemical species from many sources. The operating contact conditions in terms of load, speed and temperature also affect the nature of the tribochemical surface films. Consequently, it is very common to have the same oil additives behave in vastly different ways depending on a variety of factors. In addition, the additives and the resulting tribo films can also vary significantly over time. As a result, there are significant spatial, compositional, thickness and properties variations in tribochemical films.
There is a clear need for better understanding of the contributions and role of the tribochemical surface film to the friction and wear behavior of sliding surfaces. The traditional and usual approach of chemical analysis of the tribochemical films, while useful and perhaps needed, have not yielded fruitful results in terms of understanding the film's role in tribological performance. A new approach focusing on the material characteristics (structure) of the film is more fruitful as described hereinafter.